In a semiconductor integrated circuit, signal lines between semiconductor devices are routed along predetermined routing layers. When two signal lines run adjacent and parallel to one another for any significant length, the signals on one line, which is generally referred to as an “aggressor” line, will capacitively couple energy pulses (glitches) onto the second line, which is generally referred to as a “victim” line.
If the area of the glitch pulse on the victim line becomes large enough, the glitch pulse can cause any semiconductor devices that are downstream of this line to change state. Since this changing of state is unintended, it can cause errors in the operation of the integrated circuit. Also, if the victim line is transferring data at the time of the glitch, the glitch can cause a delay in the data, possibly causing a substantial reduction in setup time in downstream memory devices. This delay can also cause a loss of data since the downstream devices may fail to switch when they are expected to switch.
Therefore it is common for integrated circuit manufacturers to attempt to characterize the parasitic capacitances that can be formed between signal lines for a given technology through some method of estimation or measurement. These characterizations can be used during the layout process to avoid such errors.
However glitch energy is, by definition, at a very high frequency. As a result, glitch energy has historically been very difficult to measure with any kind of accuracy. Since digital circuits generally operate at the edge of the technology's frequency limit, it is very difficult to design a circuit on the same integrated circuit chip that is “faster” than the glitches the circuit is trying to measure. Another difficulty with existing glitch measurement circuits, is the fact that any circuitry that is connected to the victim line to measure the glitch adds unwanted load capacitance to the victim line and affects the accuracy of the measurement.
For example, attempts have been made to measure the peaks of the glitches on the victim line by using very high-speed comparators. The comparators compare the amplitude of each glitch pulse to a reference voltage, which is varied until the comparator fails to trip on a glitch pulse. The resulting reference voltage is then representative of the amplitude of the glitch pulses. The two difficulties discussed above (speed and unwanted capacitance on the victim line) apply to this type of measurement circuit. The comparator adds significant capacitance to the victim line, and the comparator is generally not fast enough to measure the peak of the glitch pulse.
Improved glitch energy measurement circuits are therefore desired.